Yes, I've had it. The toppings were really well-balanced--sweet caramelized onions and salty blue cheese is not an unusual combo, but they did it very well. The bun held up without getting soggy. The beef itself seemed pretty good, but since mine was overcooked (major burger faux pas) I couldn't taste the flavor as much as I would have liked. I think this may have been because my dining companion also ordered the burger, but well-done--it may have been difficult for the kitchen to time out a well-done and medium at the same table. In short, it was definitely good but not revelatory.
"... it may have been difficult for the kitchen to time out a well-done and medium at the same table."
Bern's Steakhouse in Tampa will cook a single steak for two people to two different levels of doneness. No professional kitchen -- especially one with La Cachette's pretensions -- should find it difficult to prepare separate ground-beef patties differently. So I hope you complained, although, as a former restaurant cook, I can't recommend sending something back to the kitchen for, er, further work.